Method of coating a metal base with polytetrafluoroethylene



July 12, 1960 A. CAHNE 2,944,917

METHOD OF COATING A METAL EASE WITH POLYTETRAF'LUOROETHYLENE Filed Dec. a, 1955 I NVENT Armand Ca/m ATTORNE Y METHOD OF COATING A METAL BASE WITH POLYIETRAFLUORGETHYLENE Armand Cahn, Gif-sur-Yvette, France, assignor, by mesne assignments, to Marc Gregoire, Paris, France Filed Dec. 8, 1955, Ser. No. 551,875 Claims priority, application France Dec. 13, 1954 6 Claims. (Cl. 117-49) The present invention relates to methods for coating surfaces and it particularly has for its object an improved method for coating metal surfaces with a very thin layer of polytetrafluoroethylene.

It is known that trials have been made for coating a metal surface with a layer of polytetrafluoroethylene, a product known in the trade under the name of Teflon, by interposing an intermediary layer between said metal surfaces and the Teflon layer. Such known method is diflicult to carry into practice it is costly and it can only be applied for coating surfaces having a simple shape. Moreover, the coating produced by such known method does not strongly adhere to the receiving base and, therefore, is not very durable. There lies the reason why such known method is only applied in limited cases in which the costand the life of the coating have no ma-. terial bearing. a

It is known that the difficulties encountered for coating a metal surface with a Teflon layer essentially result from the specific non-adhesivness of the Teflon.

The object of this invention is, therefore, to palliate the aforesaid drawbacks which are inherent to said known method for applying a Teflon layer. i i

More specifically, this invention has for its object to provide an improved method for coating a surface, particularly'a metal surface, and still more specially a sur: face of aluminum or aluminum alloy, with a Teflon layer, the thickness of which may vary at will over a relatively extended range; said layer is strongly anchored to the surface to be coated, does notcrack and cannot be stripped off from said surface, for instanceby means of a h-ard-edged tool.

According to the invention, a surface, generally made of metaL- to becoated with Teflon is subjected to a preparatory treatment comprising a chemical. attack with 'at least one acid, in order to produce in the outermost layer of said surface cavities or holes having a well predetermine average size, inferior to 20-30 microns. On the surface thus prepared, Teflon is deposited either as a dispersion which is then dried up, or as a powder, the resulting deposit subsequently "being fused in situ.

When the desired layer must have a thickness of ca. two hundredths of a millimeter, said layer may be produced'in a single application of Teflon; however, when thicker layers are desired, it is only necessary to repeat the same operation, while allowing each layer to dry and fusing it before depositing the next layer.

As a matter of fact, according to the invention, it has been found in practice that, provided the size of'the cavities produced in the surface to be coated by said prepara tory treatment are comprised within the rather severe range above defined, Teflon in dispersion may be, in a satisfactory manner, directly deposited on a surface thus treated and be strongly and durably bound to it.

According to the invention, it has also been found that, by utilizingan acid or a mixture of acids for the preparatory chemical attack, the cavities'producedhave the re-. quired size and have a throttled entrance, a situation 2,944,917. Patented July 12, 1960 which favors the gripping or anchoring of the film of Teflon within the cavities, and, consequently, positively provides a good anchoring (at least equivalent to a true bound) of the deposited layer of Teflon onthe surface to be coated, in spite of the well known character istics of non-adhesiveness of the Teflon, such result being due to the formation of roots which are anchored or embedded within the said cavities and are not exposed to cracking.

In order that the acid attack according to the invention produces the required cavities, the structure of the metal or non-metallic surface to be attached must have a certain heterogeneity so as to provide a differential attack producing the aforesaid cavities or holes. In opposition, some materials, such as pure copper, are uniformly attacked and the attacked surface remains smooth without cavities; in such a case, the attack by an acid or a mixture of acids does not directly produce an outer layer having cavities and upon which Teflon may be applie according to the invention.

However, in such cases when certain metals are utilized to form the base or surface to be coated, and, particularly, in the case of copper and copper alloys, a preliminary preparation of such surface comprises, according to a further feature of the invention, depositing an intermediary layer, having a thickness of ca. 2-3 hundredths of a millimeter or more, of electrolytic zinc or another suitable metal on said smooth surface, and then attacking said intermediary layer metal by at least one acid according to the invention, in order to produce the cavities required for anchoring the desired coating into said intermediary layer.

The invention may also be applied to such metal surfaces as made of enamel coating ferrous or cuprous metals, but the acid attack (preferably, with hydrofluoric acid, in that case) is rather made on the primary layer,

or mass than on the finishing layer, i.e. the outermost layer made of enamel.

It has been ascertained in practice of the invention that such attack of the enamel layer with hydrofluoric acid breaks the top or cap portion of the spherical globules of said primary layer, thus producing holesor stitches (simulating, when observed with a microscope, the tiny traces left in a mans face by small-pox pustules).

It has already been proposed to subject metal surfaces to the action of one or several acids preparatory to cerhowever, when developing the invention, it has been found that constant critical conditions, such as the selection, concentration, duration of contact and temperature of the acid or acids utilized may be predetermined, in such away that all these factors are coacting and mutually reacting, with the final result that they produce cavities having such required size that, according to the invention, the Teflon particles anchor themselves into said cavities when thereafter deposited on the base. In particular, these cavities must have a narrow or throttled entrance and a general size not exceeding 2-3 hundredths of a millimeter. Besides, a surface layer must have been disintegrated, i.e. the thickness in some regions of the part to be treated must be reduced. I

Comparative tests relating'to the invention have shown that, after having been subjected to a sand blast providing cavities of a relatively large size, i.e. exceeding 3 hunof- Teflon on the same surface,- said layer quickly works loose-and comes off, thusproving that said surface has not the anchoring and resistance, features provided by the improved proposed treatment, i.e. including the aforesaid critical conditions. i

Said critical conditions are briefly described hereinafter." Fo'r treating aluminum and aluminum alloys, and also electrolytic Zinc, hydrochloric acid having a concentration comprised between ca. 10 and 30% should beutili'z'd; for bronze and steels, it is advisable to utilize nitric acid haying a concentration in the vicinity of 10% (however, for some steels, the concentration may reach 100%); for enameled products, satisfactory results may be' obtained with hydrofluoric acid at near 15% concentration; finally, a sulfochromic mixture, that is to say a mixture of sulfuric acid and chromic acid, may also be utilized for light alloys and ferrous metals.

Before attacking the surface with acid, the attack may be, if desired, facilitated by a preparatory sand blast applied to said surface. I

In order that those skilled in the art may better understand how the present invention may be carried into practice' effect, the following illustrated examples are given.

(the starting acids are technical acids, e.g. technical hydrochloric acid at 23 Baurn).

Temperature of e the bath Time of immersion of aluminum in acid 0. F.

75 167 8, minutes. 60 140 20 minutes. 20-25 6877 a. 2 hours.

When the temperature is lower than 50 C. (122? F.), the reaction may be more easily initiated by touching'the aluminum with a piece of iron. 7

After the attack with acid, the metal is rinsed with water during 4 of an hour, then dried.

In the case of annealed aluminum, the attack should last a longer time, or nitric acid should be added to the solution of hydrochloric acid. The attack may be performed also at about 20 C. (68 F.) during 10-30 minutes, according to the annealing treatment, by utilizing the following mixture:

Percent Hydrochloric acid 30-40 Nitric acid -5-10 The surface thus prepared is then coated with Teflon in the form of an aqueous dispersion made by mixing 1 volume of water with 1 volume of the solution designated in the trade by the trade name of TD. 3, comprising Teflon granules, having a diameter approximating .5 micron, dispersed in water to which a wetting agent has been added.

For a lapse of time comprised between a few seconds and a few minutes, theaqueous dispersion of Teflon is allowed to remain in contact with the metal surface, which has already been treated as previously mentioned, thus providing on said metal surface a Teflon layer, the thickness of which is in the order of1-3 hundredths of a millimeter. The Teflon is allowed to dry and the layer is then fused ata temperature approximating 370400 C. (700-750 F.).

In this manner is produced a strongly anchored film of Teflon which has no cracks, shows no tendency to cracking in the course of time and cannot be stripped off either with the nail nor with a hard-edged tool When a thicker layer is desired, the operation is repeated, i.e. that, after a first layer of Teflon has been fused in situ, aqueous dispersion of Teflonis; again applied and the coating cycle is repeated as many times as an elementary film of Teflon is to be deposited.

The aqueous dispersion may be applied to the surface by any suitable procedure: immersion, dipping, melting, trickling or laying-down with a brush. The Teflon may also be sprayed with a gun upon the surface to be coated.

Example 2 The surface to be treated is made of an alloy designated in the trade under the name of Duralum'in (an alloy essentially comprising about 94% aluminum, about 4% copper with small quantities of manganese and magnesium).

The duralumin plate is dipped during 1'-2 minutes into 15% diluted hydrofluoric acid; the plate is washed with water, then dipped into a 40% solution of hydrochloric acid at a temperature of 20 C. (68 F.), during 2-5 minutes; the plate is then cooled down, either by taking it out of the bath from time to time, or by cooling down the bath. The plate is freed from the chlorides by dipping it into a solution, concentrated at ca. 40%, of cold nitric acid.

The plate thereby obtained has a dark grey color which may be caused to turn to a light grey by dipping the plate again during about 30 seconds into the aforesaid diluted solution of hydrofluoric acid, and then into the aforesaid bath of nitric acid during a few seconds.

Thereafter, the treatment followed is that described in Example 1, that is to say that the layer of Teflon is then applied and fused.

Example 3 'meansof a 10% solution of commercial hydrochloric acid. The duration H of the treatment is exemplified inthe table below:

' Bath temperature Duration of immersion 45 C. 167? F. sm'mute 60 0. F. 20 minutes. 20 C. 68 I 2 hours.

The layer of Teflon is subsequently applied as described in Example 1.

' Example 5 Aluminum or aluminum alloy, such as Dur-alinox or Duralumin, is attacked by a sulfochromic mixture having the following composition:

Grams Sulfuric acid I 5O Chromic acid 20 Water a I The attack is performed at 50-80" C. (:12 2-176" F.) during ca. ten minutes, and the layer of Teflon is then de osited as described in Example 1.

Example 6 For treating a bronze surface, a 12% solution of nitric acid is utilized, the immersion lasting 20 minutes at a temperature of 20 C. (68 F.). The layer of Teflon isthen deposited as described in Example 1.

Example 7 For applying a film of Teflon on. a metal capable of receiving a mechanically resistant coating, such as copper, brass, iron,vsteel, or bronze, a layer made of electrolytic zinc and having a thickness of ca. 2-3 hundredths of a millimeter may be primarily deposited on said metal.

Subsequently, said layer of electrolytic zinc is attacked with a 15% solution of hydrochloric acid, the immersion lasting 15 minutes at 20 C. (68 F.).

Teflon is then deposited as described in Example 1, or by spraying Teflon in powdered form over the attacked surface and subsequently fusing said Teflon at a temperature of about 370-400- C. (700-750 F.).

It must be mentioned that in the case of brass, iron, steel, bronze, the preparatory attack with an acid or a suitable mixture of acids may be realized, without previously depositing electrolytic zinc, while such zinc deposit is necessary for copper parts, on account of the fact, as previously explained, that copper is uniformly attacked by acids.

Example 8 l The metal surface to be treated is a stub steel (with 1% carbon, .5% chromium and .l% vanadium).

The attack is performed with a 10% solution of commercial nitric acid during 6 minutes at 20 C.

In a modified form of the treatment according to the invention, the attack may be performed in two phases, namely.

First phase: attack with a solution of commercial nitric acid at 100% concentration at 20 C. (68 F.) during about 30-60 seconds; then 7 Second phase: attack, also made at 20 C..(68 F.), with a solution of commercial nitric acid at 10% concentration during minutes.

Teflon is deposited as described in Examples 1 and 2 (Teflon in powdered form).

- Example 9 A steel, of the so-called B.T.H. type (with 1% carbon and 1.1% manganese), is treated with a 10% solution of nitric acid at 20 C. (68 F.), in two phases: attack during 10 minutes at 20 C. (68 F.), then, a brush cleaning and second attack at 20 C. (68 F.) during 5 minutes. I

Teflon is deposited as previously described.

Example 10 For treating half-cold-hardened low carbon steel having .10% carbon and .4-.6% manganese, the attack is made with nitric acid at 20 C. (68 F.) during 12 minutes, the acid concentration being 10%.

Teflon is deposited as previously described.

Example 11 Example 12 For treating iron, steel or another ferrous metal, or a copper alloy, the preparatory treatment comprises sand blasting the surface with very fine sand; then, a first layer is deposited, which comprises a Teflon mixture,

6 V such as the aforesaid TD. 3 dispersion which is diluted at 50% and containing, in volume, 5% of a sulfochromic mixture, the composition of which is'indicated below:

. I Grams Sulfuric acid 50 Chromic acid 20 Water 150 After said first layer has been fused in situ', other layers of pure Teflon may be deposited; l1owever, with such an attack, the whole deposited layer may have only a small'thickness.

Example 13 Parkerised or bonderised iron, that is to say iron which has been immersed for about one hour or ten minutes,

respectively into a boiling solution of iron and manganese phosphates, is covered with a first layer as described in Example 12, but containing only 2% (instead of 5%), in volume, of the sulfochromic solution.

Then, when the deposited layer has been dried, it is fused.

Example 14 A metal surface, made, for instance, of steel or of an aluminum alloy, is directly treated with a first layer comprising a Teflon dispersion mixed with 10% of the sulfochromic solution indicated below:

Grams Sulfuric acid 50 Chromic acid -20 Water 150 Thereafter, layers of pure Teflon may be deposited.

The results achieved with such a procedure are not so satisfactory than when the metal has been previously attacked with an acid because when, later, the coated surface comes in contact with boiling water, the coating thus produced cannot resist.

' Example 15 A plate made of aluminum or duralinox is firstly at tacked at 60 C. F.) for a few minutes with the followingmixture: V V

Grams Sulfuric acid 50 Chromic acid 20 Water V Commercially pure hydrochloric acid 10 Then, the plate is subjected to a second attack, at 60 C. (140 F.) for a few minutes with the following solution:

, Grams Sulfuric acid 50 Chromic acid 20 Water 150 Pure Teflon is then deposited as previously-described.

Example 16 A small plate made of an enameled ferrous or cuprous metal is dipped into a 15 solution of hydrofluoric acid for 25 minutes at 20 C. (68 F.).

The acid attack transforms the globular mass of enamel into a sponge, by breaking off the top portions of the enamel globules.

It is advantageous to attack the primary layer or masslized, .it is advantageous to slightly increase the abovementioned acid concentrations, since the concentrations indicated inlsaid examples areparticularly suitable when the acid or acids are directly brought in contact with the metal, as, for instance, by dipping.

In all examples, the thickness of the whole layer may be increased by coating, melting, trickling, or by utilizing a light spraying gun.

When a thick-Teflon coating is desired, it is advantageous to f or m relatively .thin elementary layers one after the other, each deposited elementary layer being fused before the next elementary layer is deposited.

In a particular form of carrying the invention into practice, the concentration in the successively deposited elementary layers of Teflon increase from one elementary layer to the next one, so that advantage is taken of the strong anchoring produced by the diluted innermost elementary layer of Teflon laid on the metal base as well as of the high non-adhesiveness or hydrophobic characteristic given to the whole layer of Teflon by the more concentrated outermost elementary layer of Teflon; In this manner, for producing the same coating having a predetermined thickness, the number of necessary ele- 1 tion costs.

The coating method according to the invention provides a Teflon layer the Whole thickness of which may be, for instance, comprised between 2 and hundredths of a millimeter, and said layer does not peel off, nor crack, and cannot be stripped off with the nail or a cutting tool. Said layer can stand the squaring test, which comprises tracing two perpendicularly disposed groups of lines on a Teflon coated surface by means of a cutting tool so as to form a corresponding pattern indentedinto the exposed surface; finally, the film of Teflon remains strongly anchored to the treated surface up to the sublimation temperature of the Teflon deposited.

The accompanying drawing diagrammatically shows in cross-section various forms of cavities provided in a surface by an attack treatment according to the invention, together with .a substantially corresponding measuring scale. i

While only the main objects of the invention have been described and only some examples for carrying the said invention into practice have been given in the above specification, it will be understood, of course, that the present invention is not limited thereto, since many modifications may be made and the appended claims intend to cover such modifications as fall within the true spirit and scope of the invention.

I claim:

1. A method for coating a base selected from the group consisting of iron and aluminum with a continuous film of polytetrafiuoroethylene anchored into said base, comprising the steps of providing anchoring cavities having an average size less than 30 microns and a throttled entrance in said base by attacking it with at least one acid, subsequently depositing an aqueous dispersion consisting essentially of polytetrafluoroethylene thereon in a succession of elementary layers, and drying and fusing each layer in situ before the next is deposited, the concentration of polytetrafluoroethylene in each successive layer being greater than thatin the layer previously deposited.

2. A method for coating a base of aluminum-based metal with a continuous film consisting essentially of polytetrafiuoroethylene anchored into said base, comprisingthe steps of providinganchoring cavities having an average size less than 30 microns and'a throttled entrance in said base by attacking it with a 10% solution of hydrochloric acid ,in water for a period of between 8 minutes and two hours at a temperatureib'etween 20 C. and C., the maximum temperature being used for .the minimum time and the maximum time for the mum temperature, the timeemployed for any particular attack varying upward from the lower limit to the upper limit as the temperature employed varies downward from its upper limit toward its lower limit, and subsequently depositing the polytetrafluoroethylene thereon.

3. A method for coating an aluminum based metal with a continuous film consistingessentially ,of polytetrafluoroethyleneanchored into said base comprising the steps of providing anchoring cavities having an average size less than 30 microns and a throttled entrance in said base by first attacking the metal with hydrochloric acid and then with a sulfochromic solution containing, by weight, substantially 5 parts of sulfuric acid, ,2 parts of chromic acid, and 15 parts of water, at a temperature between 3b9ut50 C, and C., depositing the polytetrafluoroethylene on said base andjfus'ing it in situ.

4. A method of coating a ferrous metal base with a film consisting essentially polytetrafiuoroethylene which comprises the steps of first applying to said base a thick enamel coating, attacking the enamel coating with a 15% solution of hydrofluoric acid, to produce cavities having an average size less than 30 microns and a throttledentrance depositing the polytetrafluoroethylene thereon, and fusing said polytetrafluoroethylene in situ.

5. A method of coating a metallic base consisting primarily of aluminum with a continuous film consisting essentially of polytetrafluoroethylene anchored into said base, which comprises the steps of creating anchoring cavities less than 30 microns in diameter andhaving a throttled entrance in said base by attacking it with -hydrochloric acid, subsequently depositing the polytetrafluoroethylene on said base and fusing the polytetrafluoroethylene in situ. P g V 6. A methodof coating a metallic *base consisting primarily of aluminum with a continuous film consisting essentially of polytetrafiuoroethylene anchored to said base, which comprises, the'steps of creating anchoring cavities less than 30 microns in diameter and not varying in depth in excess of 30 microns in said base and having a throttled entrance, by attacking it with hydrochloric acid, subsequently depositing the polytetrafluoroethylene on said base and fusing the polytetrafluoroethylene in situ, said material having a minimum thickness which is not less than 20 microns.

References Cited in the file of this patent UNITED STATES PATENTS 2,199,227 Marks Apr. 30, 1940 2,562,117 Osdal July 24, 1951 2,567,162 Sanders Sept. 4, 1951 2,691,814 Tait Oct. 19, 1954 2,731,360 Love Jan. 17, 1956 2,762,694 Newman Sept. 11, 1956 2,776,918 Bersworth Jan. 8, 1957 2,777,783 Welch Jan. 15, 1957 FOREIGN PATENTS 657,085 Great Britain Sept. 12, 1951 

1. A METHOD FOR COATING A BASE SELECTED FROM THE GROUP CONSISTING OF IRON AND ALUMINUM WITH A CONTINUOUS FILM OF POLYTETRAFLUORETHYLENE ANCHORED INTO SAID BASE, COMPRISING THE STEPS OF PROVIDING ANCHORING CAVITIES HAVING AN AVERAGE SIZE LESS THAN 30 MICRONS AND A THROTTLED ENTRANCE IN SAID BASE BY ATTACKING IT WITH AT LEAST ONE ACID, SUBSEQUENTLY DEPOSITING AN AQUEOUS DISPERSION CONSISTING ESSENTIALLY OF POLYTETRAFLUOROETHYLENE THEREON IN A SUCCESSION OF ELEMENTARY LAYERS, AND DRYING AND FUSING EACH LAYER IN SITU BEFORE THE NEXT IS DEPOSITED, THE CONCENTRATION OF POLYTETRAFLUOROETHYLENE IN EACH SUCCESSIVE LAYER BEING GREATER THAN THAT IN THE LAYER PREVIOUSLY DEPOSITED. 